1. Field of the Invention
The present invention relates to a light-emitting element, a display device, a light-emitting device, an electronic device, and a lighting device each of which includes an organic compound as a light-emitting substance.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Since such a light-emitting element is of self-light-emitting type, the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. In addition, it is also a great advantage that a display including such a light-emitting element can be manufactured as a thin and lightweight display. Furthermore, very high speed response is also one of the features of such an element.
Since a light-emitting layer of such a light-emitting element can be formed in the form of a film, light emission from a planar surface can be achieved. Therefore, large-area light sources can be easily formed. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applied to lighting devices and the like.
In the case of an organic EL element in which an EL layer containing an organic compound as the light-emitting substance is provided between a pair of electrodes, application of a voltage between the pair of electrodes causes injection of electrons from the cathode and holes from the anode into the EL layer, and thus a current flows. By recombination of the injected electrons and holes, the organic compound having a light-emitting property is excited and provides light emission from the excited state.
The excited state of an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state (S*) is referred to as fluorescence, and light emission from the triplet excited state (T*) is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S*:T*=1:3. Therefore, a light-emitting element including a phosphorescent compound capable of converting the triplet excited state into light emission has been actively developed in recent years.
As a result, many phosphorescent substances have been developed and widely used, and red to green phosphorescent substances are already at the stage of practical use. However, blue phosphorescent substances have problems with their characteristics, especially lifetimes, and require further research and development for practical use.
Patent Document 1 discloses, as a phosphorescent compound emitting phosphorescence with a short wavelength, an iridium complex in which an imidazole derivative is a ligand.